Paramagnetic Relaxation Agents for Enhancing Temporal Resolution and Sensitivity in Multinuclear FlowNMR Spectroscopy.

Sensitivity in FlowNMR spectroscopy for reaction monitoring often suffers from low levels of pre-magnetisation due to limited residence times of the sample in the magnetic field. While this in-flow effect is tolerable for high sensitivity nuclei such as 1 H and 19 F, it significantly reduces the signal-to-noise ratio in 31 P and 13 C spectra, making FlowNMR impractical for low sensititvity nuclei at low concentrations. Paramagnetic relaxation agents (PRAs), which enhance polarisation and spin-lattice relaxation, could eliminate the adverse in-flow effect and improve the signal-to-noise ratio. Herein, [Co(acac)3 ], [Mn(acac)3 ], [Fe(acac)3 ], [Cr(acac)3 ], [Ni(acac)2 ]3, [Gd(tmhd)3 ] and [Cr(tmhd)3 ] are investigated for their effectiveness in improving signal intensity per unit time in FlowNMR applications under the additional constraint of chemical inertness towards catalytically active transition metal complexes. High-spin Cr(III) acetylacetonates emerged as the most effective compounds, successfully reducing 31 P T1 values four- to five-fold at PRA concentrations as low as 10 mM without causing adverse line broadening. Whereas [Cr(acac)3 ] showed signs of chemical reactivity with a mixture of triphenylphosphine, triphenylphosphine oxide and triphenylphosphate over the course of several hours at 80° C, the bulkier [Cr(tmhd)3 ] was stable and equally effective as a PRA under these conditions. Compatibility with a range of representative transition metal complexes often used in homogeneous catalysis has been investigated, and application of [Cr(tmhd)3 ] in significantly improving 1 H and 31 P{1 H} FlowNMR data quality in a Rh-catalysed hydroformylation reaction has been demonstrated. With the PRA added, 13 C relaxation times were reduced more than six-fold, allowing quantitative reaction monitoring of substrate consumption and product formation by 13 C{1 H} FlowNMR spectroscopy at natural abundance.

Single-Molecule Conductance Behavior of Molecular Bundles.

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5'-bis(methylthio)-2,2'-bipyridine (L1) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) (L2) to give two monometallic complexes, Fe-1 and Co-1, and two bimetallic helicates, Fe-2 and Co-2. Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires.

Multi-nuclear, high-pressure, operando FlowNMR spectroscopic study of Rh/PPh3 - catalysed hydroformylation of 1-hexene.

The hydroformylation of 1-hexene with 12 bar of 1 : 1 H2/CO in the presence of the catalytic system [Rh(acac)(CO)2]/PPh3 was successfully studied by real-time multinuclear high-resolution FlowNMR spectroscopy at 50 °C. Quantitative reaction progress curves that yield rates as well as chemo- and regioselectivities have been obtained with varying P/Rh loadings. Dissolved H2 can be monitored in solution to ensure true operando conditions without gas limitation. 31P{1H} and selective excitation 1H pulse sequences have been periodically interleaved with 1H FlowNMR measurements to detect Rh-phosphine intermediates during the catalysis. Stopped-flow experiments in combination with diffusion measurements and 2D heteronuclear correlation experiments showed the known tris-phosphine complex [RhH(CO)(PPh3)3] to generate rapidly exchanging isomers of the bis-phosphine complex [Rh(CO)2(PPh3)2] under CO pressure that directly enter the catalytic cycle. A new mono-phosphine acyl complex has been identified as an in-cycle reaction intermediate.